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“Water vapour in the atmosphere can change phase, which leads to more clouds, and greater cloud cover means that more sunlight is reflected straight out of the atmosphere. Crude calculations suggest that the two effects approximately balance each other, and that water vapour does not
have a strong feedback mechanism in the Earth’s climate.”

Unfortunately it doesn’t have a reference. And we need more than ‘crude’ calculations. But it certainly doesn’t support the high-feedback estimates used in some many climate change models.

John,
Thanks for the link. I made a paper water molecule while drinking my coffee and started thinking about water dimers after not finding a new way to vibrate a single water molecule. Wasn’t expecting to, but it was fun. :)
You can imagine my joy when they were mentioned several paragraphs later as an unknown contribution. I had forgotten the term but remembered the concept from a chemistry class oh so long ago.

What a thing to sick some super bright grad students on! From the article:

“…, it is well known that at low temperature pairs of water molecules will stick together to form a weakly bound molecule known as a dimer. The absorption properties of the water dimer at visible wavelengths will be different from those of a single water molecule, but these remain to be characterized. Furthermore, it has so far proved impossible to determine the proportion of atmospheric water molecules that are present as dimers in either laboratory or atmospheric measurements.”

Liquid water absorbs IR quite strongly. I engaged in a small debate on Real Climate on the subject some time back. Thermal IR can’t penetrate more than a centimeter or so if I recall. Of course in something real-world like the ocean the heat penetrates rather more from mixing. So this is primarily what happens with clouds. The thicker ones, at least, absorb practically all IR at all frequencies and this warms the cloud and the atmosphere within it. But on top of the cloud a lot of IR escapes.